In the hydraulic mechanisms of construction machines such as the power shovel, cylinder rods inclusive of piston rods, etc. are conventionally used. Such rods, slidable against cylinders, must satisfy several characteristic requirements such as high hardness, high wear resistance and high dimensional accuracy inclusive of surface accuracy. Therefore, rods for cylinder use are generally fabricated by applying hard chrome plating to iron-based substrate rods.
However, the hard chrome plating layer so formed, unlike other electroplating layers, tends to develop various defects such as cracks, pinholes and pits even when its thickness is as great as the order of 100 .mu.m. Therefore, despite its various meritorious characteristics, the rod carrying a hard chrome plating layer is more or less readily corroded and rusted. Particularly in the presence of chloride ions, e.g. sodium chloride, or acids or under high-temperature, high-humidity field conditions, the rod undergoes serious corrosion due to the numerous defects. As the rod is corroded, it is damaged on repeated sliding against the cylinder to cause hydraulic oil leaks.
To prevent corrosion of hard-chrome-plated members, it is common practice to apply a rust inhibitor to the surface of the members and let the inhibitor penetrate into the defects but this practice is hardly a radical solution to the problem. For the prevention of corrosion of hard-chrome-plated members, it is also practiced to form a plating layer as thick as about 0.05 to 0.1 mm. However, increasing the thickness of the plating layer results in decreased productivity.
Japanese Patent Publication No. 3-14913 (JP-B-3 (1991)-14913) describes a process for applying a high-quality mirror-surface chrome plating which comprises forming a plating layer approximately twice as thick as a finished plating layer on an iron-based work using a microcrack type chrome plating bath, baking the plating layer under rugged conditions (at a temperature 40.degree.-60.degree. C. higher than the upper critical standard field operating temperature for 40-50 hours), removing 40-50% of the plating layer by buffing, baking the buffed layer under mild conditions (at a temperature 10.degree.-20.degree. C. higher than the upper critical standard field operating temperature for 20-30 hours) and finishing the surface by buffing or polishing.
This method, however, is not only uneconomical because a very thick plating layer must be first formed and 40-50% of the layer be then removed but also low in productivity because these operations are time-consuming. Furthermore, the two baking steps required are also time-consuming. Therefore, the productivity of drums or rolls, for instance, is seriously low. In addition, corrosion resistance is not a parameter considered in the above prior art literature.
Japanese Patent Application No. 60-33369 (JP-A-60 (1985)-33369) discloses a method of inhibiting corrosion which comprises chrome-plating a metal work, heating the same at a temperature of 120 +/- 10.degree. C.3 for 5 to 30 minutes to remove the hydrogen gas evolved in the course of plating and coating the plating layer with an anticorrosive composition comprising a mixture of 20 to 80 weight % of an unsaturated polyester and 80 to 20 weight % of diallyl phthalate. The resultant metal work, however, is insufficient in corrosion resistance as shown in Comparative Example.
In Japanese Patent Application No. 4-160197 (JP-A-4 (1992)-160197), a method of improving the corrosion resistance of cylinder rods which comprises subjecting a cylinder rod to hard chrome plating and, then, to baking. The present invention has for its object to further improve the corrosion resistance of hard-chrome-plated iron-based members such as cylinder rods.